1. Field of the Invention
This invention relates to the subsea environment and specifically to the protection of associated subsea structures such as wellheads, caissons, mud line wells, trees and flowline hubs.
2. Background of the Invention
A typical subsea scenario requiring the installation of a cap commences when an operation, such as drilling a subsea well, is suspended or terminated. During the preceding drilling operation a foundation casing is washed down and inserted into the seabed such that approximately ten feet of the casing protrudes upward from the seabed. A hole is then drilled through the primary foundation and down to a predetermined depth. The next section of pipe is then run into the drilled hole and landed inside the previous foundation casing. On top of this section of pipe typically is a wellhead of various inner and outer diameters. The top of the wellhead is now about thirteen feet above the sea floor. This last section contains a seal pocket that will allow it to seal when attached to a Blow-Out-Protector (“BOP”) or other sealing or connecting equipment. This seal pocket can be capable of containing pressures at or in excess of 15,000 psi.
The drilling rig then runs a BOP device that will latch onto the wellhead creating a sealed conduit from the subsea structure back up to the drilling rig where the pressure can be observed and controlled. Between the section and the BOP is a gasket. This gasket is in contact with both the wellhead and the BOP and creates a seal. Any abnormalities in the wellhead seal pocket could keep the seal from holding. Therefore, protecting this exposed seal pocket in the wellhead becomes very important in the life of the well and/or equipment.
The well is drilled for exploration or development. Once the well has been drilled and the rig is ready to move off location or move to another well, the BOP is disconnected from the wellhead. The seal pocket on the wellhead is now exposed and vulnerable to dropping and/or foreign objects. A subsea well can cost several millions of dollars to drill. Leaving the wellhead exposed to damage from falling objects or other intruders could result in loss of the seal integrity and thereby render the wellhead useless.
When a wellsite is abandoned or temporarily suspended, it is desirable to protect the wellhead. During the installation of a subsea flow line, the drilling of a subsea well, the drilling of a mudline suspension well or the installation of a subsea tree, it may be necessary to suspend the operation due to inclement weather or requirement of additional equipment. When the operations have been suspended or completed, the end or top of the pipe or equipment has a seal area or a profile that needs to be protected from foreign objects, damage and/or marine growth. These seals, surfaces and profiles have varied outer and inner diameters, shapes and lengths. They also may contain seal pockets or integral components that need to be protected from corrosion and/or marine growth.
Because of the potential for enormous loss of time and money and the need for protection on multiple structure profiles, it is desirable to have a cap for a subsea structure that has the following characteristics:    (1) Protects an open subsea structure from falling matter and foreign objects;    (2) Fits on many profile-dissimilar structures (wellheads, caissons, trees, flowline hubs, etc.) without modification;    (3) Installable and/or removable by an ROV and/or diver or surface/rig mounted winch;    (4) Sufficient sealing characteristics derived by weight of cap and pressure of surrounding seawater if cap is required to contain corrosive-inhibiting and/or other fluids such as marine-growth-inhibitors;    (5) Installable to the structure without locking to the structure;    (6) Removable from the structure without unlocking from the structure;    (7) No orientation required for installation other than lowering the cap onto the structure;    (8) No external pressure requirements to accomplish its job other than pressure from surrounding seawater;    (9) Moveable from one subsea structure to the next without resurfacing;    (10) Easily removeable/reinstallable for structure cleaning operations;    (11) Reusable without replacing any parts; and    (12) Inexpensive to manufacture and maintain.
Current methods of protecting an exposed wellhead or other structure include the capping of a structure by a device such as disclosed in U.S. Pat. No. 5,107,931 (“the '931 patent”). Although the '931 patent meets one of the basic requirements (#1: protects the open subsea structure from falling matter and foreign objects), it fails to meet or even disclose any of the other requirements (#2–11) listed above.
For example, the cap of the '931 patent is designed specifically to latch to a wellhead to form a seal (fails #4, 5, 6) and must mate exactly with existing elements of that wellhead. This means that it will not work on any other type of subsea structure that does not have the identical mating elements (fails #2). Additionally, the cap of the '931 patent is designed to work with a drill string tool only (fails #3) and requires orientation (fails #7) and pressure from the tool to latch to and unlatch from the structure (fails #8). Because pins located in the wall of the '931 cap are sheared off during operation, new pins must be retrofitted to prepare the cap for another job (fails #9, 10 and 11). Because the '931 cap requires a complex latching and release mechanism to install and remove the cap, it will not be inexpensive to manufacture or maintain (fails #12).
One of the most prominent characteristics of currently available protective coverings for subsea structures is that the size and shape of the covering must be closely matched to the size and shape of the subsea structure to enable the covering to mate with and latch to the structure. This design constraint means that there must be different coverings for different applications and the mating/latching requirements mean that the covering is much more expensive to manufacture and maintain.
In general, the approach of the prior art has been to focus on a particular aspect of protection that is very structure-dependent, costly to manufacture and complex to operate. None of the prior art meets the requirements set forth above. It would, therefore, be a significant advancement in the art and it is an object of the present invention to provide an improved cap that is simple to manufacture, install and remove and that meets all of the desired requirements cited above.